Because of its ready availability, low cost and potential for reducing particulate emissions, natural gas is a promising candidate for fuelling diesel engines. Methods that can be used to convert a diesel engine (a compression-ignition engine) to consume natural gas fall into three categories. One is to convert the engine to a stoichiometric or lean-burn spark-ignition engine. As a second option, the engine can be converted to natural gas using a "dual-fuel" technology, in which the natural gas is mixed with all of or with a portion of the intake air and is ignited by diesel fuel injected at the end of the compression stroke. A third method is the direct injection of the natural gas fuel in the combustion chamber, with a source of ignition. The preferred method, as discussed below, is the direct injection method because it is the only method which preserves the inherent favourable operating characteristics and high efficiency of diesel engines.